This invention relates to slab gel electrophoresis, and particularly to molds for slab gels and methods for forming slab gels using such molds.
Slab gels and their use in electrophoresis are widely disclosed in the literature. Recent examples are U.S. Pat. Nos. 4,574,040, issued Mar. 4, 1986 (DeLony et al.) and 4,663,015, issued May 5, 1987 (Sleeter et al.). In systems such as those described in these patents, the gels are formed by pouring a gel-forming liquid into the space between two flat glass plates whose spacing is maintained by spacer strips placed between the plates along two opposing edges, with clamps along each of these edges holding them together like a "sandwich." The spacer strips are smooth so that a seal is formed under the pressure of the clamp, preventing leakage of either the gel-forming solution during gel casting or the buffer solution during electrophoresis.
In gels having a constant thickness and constant gel porosity along the direction of migration, the relationship between the molecular weight of the species being separated and their mobility through the gel is logarithmic. As these species separate into bands and migrate through the gel, the result is a narrow spacing of bands near the starting end of the gel and a relatively wider spacing toward the far end, the end toward which the bands are migrating. This makes detection and analysis difficult in many cases, particularly when many species are to be separated, such as in nucleic acid sequencing.
To counter this effect, gradient gels and wedge-shaped gels have been used in place of the constant gels referred to above. Although gradient gels have been used successfully, they are troublesome to pour, and particularly so in the case of gels of extreme length. Wedge spacers, on the other hand, are easier to use, the thickest end of the spacers being placed at the end away from the starting end of the gel to decrease band mobility. Wedge spacers are difficult to manufacture, however, since they require a high degree of dimensional control for purposes of uniformity and gel reproducibility, as well as a high degree of surface smoothness to prevent leakage. Wedge spacers currently available are machined to their final shape, which makes it difficult to meet these requirements.